This invention relates to dispersant compositions for treating oil slicks on water.
Pollution of seawater by oil (crude oil or fractions of crude oil) occasioned by accidents, off-shore drilling operations, discharge of ballast water or spillage from oil tankers, results in the formation of a continuous film or slick of oil which tends to spread. This oily film is undesirable because it constitutes a barrier to the transfer of air and light from the atmosphere into the water, and without air and light the water cannot support marine life.
The more generally employed method for dealing with such pollution problems consists in treating the oil film with surface-active agents which disintegrate the cohesive oily film into droplets and disperse the droplets into the water column to a depth of several meters under the sea surface. The film is thus broken and there is again a transfer of air and light from the atmosphere to the water. Moreover, when oil is formed into droplets, digestion of the oil by the micro-organisms that live in seawater is much easier.
Some surface-active agents are highly effective as oil slicks dispersants, but they are very toxic towards aquatic flora and fauna and their use results in severe damage to marine life. On the otherhand, surface-active agents that have a lower degree of toxicity are often ineffective as oil dispersants in seawater. In fact, they are too quickly washed out by water with the result that the oil droplets coalesce, re-agglomerate and again form a slick.
Due to the problems noted above, highly effective surface-active agents are generally used in admixture with a solvent, particularly a solvent having a low degree of toxicity, such as paraffinic and cycloparaffinic hydrocarbons and lower alcohols. The use of a solvent presents many advantages: it decreases the pour point of the composition, it reduces the viscosity of the oil forming the slick, it acts as a diluent for the surface-active agent which is more readily and uniformly distributed into the oil slick, and it reduces the time required to mix the dispersant agent and the oil.
Another problem which arises from the use of surface-active agents as oil spill dispersants is their water-solubility. Some of these agents dissolve too quickly in water before they have time to penetrate the oil slick. For this reason, it has been suggested to use dispersant compositions containing at least two surface-active agents. Better dispersion results from the use of blends of surface-active agents whose proportions are selected so that the hydrophilic-lipophilic balance (or HLB) falls between defined limits.
The trend therefore is to use blends of surface-active agents in admixture with a solvent. British Patent No. 1,404,684 describes such a composition in which the oil slick dispersant comprises a blend of fatty acid polyoxyalkylene glycol esters and fatty acid sorbitan esters (HLB of the blend: from 9 to 10.5), dissolved in a hydrocarbon solvet containing less than 3 weight percent aromatics. The solution may contain from 5 to 50 weight percent of the surfactant blend, a proportion of 8 to 10 weight percent being particularly preferred. Other patents describe the use of compositions comprising a fatty acid sorbitan monoester and a polyalkylene oxide adduct of fatty acid sorbitan monoester (British Patent No. 1,255,394) or the use of these compounds in admixture with 25 to 35 volume percent (exclusive of solvent) of a 75% aqueous solution of dialkyl sulfosuccinate salt (U.S. Pat. No. 3,793,218). The best results in dispersing oil slicks were obtained from formulations containing 40 percent of a blend of surface-active agents and about 60 percent of an isoparaffinic hydrocarbon solvent.
However, problems may occur with dispersants containing a blend of surface-active agents and a solvent. The HLB of some compositions must fall within limits that are too narrow. Others require agitation, that means a rough sea, in order to achieve a disintegration of the oily film.
There is thus a need for improved dispersant compositions whose effectiveness is not appreciably affected by the above factors.
The object of this invention is to provide dispersant compositions for treating oil slicks on seawater that fulfill the following conditions:
have a high content of surface-active agents, PA1 have a fast and long-lasting dispersion action, PA1 be effective as well on calm sea (low energy mixing) as on rough sea, PA1 exhibit an effectiveness which does not depend on a narrow critical HLB, PA1 be a homogeneous limpid liquid, free from insoluble material. PA1 have an adequate viscosity, allowing the composition to be sprayed from boats or from aicraft, PA1 be non-toxic and biodegradable, PA1 be easily manufactured and at low cost.